Synthetic-Aperture Imaging Ladar

Walter F. Buell, Nicholas J. Marechal, Joseph R. Buck, Richard P. Dickinson, David Kozlowski, Timothy J. Wright, and Steven M. Beck

Aerospace has been developing a remote-sensing technique that combines ultrawideband coherent laser radar with synthetic-aperture signal processing. The goal is to achieve high-resolution two- and three-dimensional imaging at long range, day or night, with modest aperture diameters.

Conventional optical imagers, including imaging radars, are limited in spatial resolution by the diffraction limit of the telescope aperture. As the aperture size increases, the resolution improves; as the range increases, resolution degrades. Thus, high-resolution imaging at long ranges requires large telescope diameters. Imaging resolution is further dependent on wavelength, with longer wavelengths producing coarser spatial resolution. Thus, the limitations of diffraction are most apparent in the radio-frequency domain (as opposed to the optical domain, for example).

A technique known as synthetic-aperture radar was invented in the 1950s to overcome this limitation: In simple terms, a large radar aperture is simulated or synthesized by processing the pulses emitted at different locations by a radar as it moves, typically on an airplane or a satellite. The resulting image resolution is characteristic of significantly larger systems. For example, the Canadian RadarSat–II, which is slated to fly at an altitude of about 800 kilometers, has an antenna size of 15 X 1.5 meters and operates at a wavelength of 5.6 centimeters. Its real-aperture resolution is on the order of 1 kilometer, while its synthetic-aperture resolution is as fine as 3 meters. This resolution enhancement is made possible by recording the phase history of the radar signal as it travels to the target and returns from various scattering centers in the scene. The final synthetic-aperture radar image is reconstructed from many pulses transmitted and received during a synthetic-aperture evolution time using sophisticated signal-processing techniques.

The SAIL concept: A platform with a transmit/receive module evolves a synthetic aperture by moving some distance while illuminating a target some distance away and receiving the scattered light. The transmitted light has a wide- bandwidth waveform imposed upon it. The illuminating spot size at the target is the same as the diffraction-limited resolution of the transceiver optic. This "real-aperture" resolution is proportional to the wavelength of the transmitted light and the range to the target, and inversely proportional to the transceiver optic size. The synthetic-aperture imaging resolution along the direction of travel is determined by the diffraction limit of the synthetic aperture and is proportional to change in azimuth angle as seen by an observer at the target and therefore proportional to the length of the evolved synthetic aperture. The range resolution is the speed of light divided by twice the transmit bandwidth.

Aerospace is investigating ways to apply the techniques and processing tools of radio-frequency synthetic-aperture radars to optical laser radars (or ladars). There are several motivations for developing such an approach in the optical or visible domain. The first is simply that humans are used to seeing the world at optical wavelengths. Optical synthetic-aperture imagery would potentially be easier for humans to interpret, even without specialized training. Second, optical wavelengths are around 10,000 times shorter than radio-frequency wavelengths and can therefore provide much finer spatial resolution and much faster imaging times. Finally, unlike passive imagery, ladar, like radar, provides its own illumination and can generate imagery day or night. Despite some early laboratory experiments, optical synthetic-aperture imaging has not been realized before because of the extreme difficulty of maintaining comparable phase stability and wide bandwidth waveforms in the optical domain, especially at the high laser powers required for long-range operation. Advances in both laser technology and signal processing are now at a stage where such systems may be realizable.

The Aerospace Approach

The Aerospace experimental approach is called synthetic-aperture imaging ladar, or SAIL. The SAIL concept is best envisioned in terms of a platform with a transmit/receive module moving on a trajectory, illuminating a target and receiving the scattered light. The spot size at the target is determined by the diffraction limit of the transceiver optic; this corresponds to the imaging resolution for a conventional imager. The imaging resolution in the direction of sensor motion is determined by the diffraction limit of the synthetic aperture, a function of the synthetic-aperture length developed during a period of flight. The resolution in the range direction is determined by the bandwidth of the transmitted waveform.

Unlike the resolution of a real-aperture imager, the attainable resolution of a synthetic-aperture system is essentially independent of range. Of course, nothing comes for free, and SAIL operation at longer ranges requires greater laser power.

Some of the earliest "synthetic-aperture" experiments in the optical domain were performed in the late 1960s and demonstrated inverse synthetic-aperture imaging of a point target simply swinging on a pendulum ("inverse" in the sense that the target, rather than the platform, is in motion.) Recent efforts at MIT's Lincoln Laboratory have included the use of an Nd:YAG microchip laser to demonstrate inverse synthetic-aperture imaging in one dimension with conventional diffraction-limited imaging in the other dimension (using a high-aspect-ratio aperture) to produce two-dimensional images. The Naval Research Laboratory has also demonstrated fully two-dimensional inverse SAIL imaging of a translated target using 10 nanometers of optical bandwidth at a wavelength of 1.55 micron. The SAIL images obtained at Aerospace represent the first true optical synthetic-aperture images made using a moving transmit/receive aperture, as well as the first SAIL image from a diffuse scattering target.

Experimental Setup

The Aerospace experiments employ 1.5-micron semiconductor and fiber laser transmitters and related components commonly found in the commercial fiber-optic telecom industry. This approach was motivated by several considerations. To begin with, researchers were interested in SAIL system design, image formation, and phenomenology—not component development. Operation at 1.5-micron wavelengths allows the researchers to apply both the commercial telecom component base and expertise gained in other photonics research activities at Aerospace. Furthermore, the 1.5-micron wavelength is in the nominally eyesafe wavelength regime and relatively close to the visible region of the spectrum (where people are used to seeing the world). Finally, researchers sought to maintain scalability to long-range operation without significant technology or design changes, and 1.5-micron fiber laser technology is compact, efficient, relatively robust, and potentially scaleable to high-power operation.

The SAIL image formation process requires measurement of the phase history of the returned ladar signals throughout the synthetic-aperture formation time, just as in synthetic-aperture radar. This is accomplished using coherent (heterodyne) detection, wherein the return signals are optically mixed with a stable local oscillator. The local oscillator acts as an onboard optical phase reference, and when the return signal and local oscillator are superimposed on a photodetector, the resulting electrical signal contains information about the phase and frequency difference between them. In Aerospace experiments, the local oscillator is derived from the same laser as the transmitted pulses and has the same waveform imposed upon it (see sidebar, Signal Processing for SAIL).

System schematic. The signal is fed into an optical splitter with one percent directed to a molecular wavelength reference (labeled "HCN," for hydrogen cyanide) to ensure that each chirped pulse begins at the same optical frequency. The remaining signal passes through another optical splitter with one percent directed to the local oscillators and the reference arm. The main part of the transmitted light passes through the optical circulator and on to the transceiver optics, which direct the laser beam to the target. The reflected signal passes back through the transceiver optics and an optical circulator and is mixed with the optical local oscillator for heterodyne conversion at the detector. The electrical signal from the detector is then fed to an analog-to-digital converter and on to the computer. The transceiver aperture is translated in 50-micron steps with a stepping translation stage to create the synthetic aperture, and one frequency-swept pulse is emitted at each transceiver position.

The first stage of development focused on a laboratory-scale demonstration, which permits easy system modifications and allows researchers to build knowledge of image formation and phenomenology. Because the ranges involved are fairly short (roughly 2–3 meters) and the targets quite small (from a few millimeters to a few centimeters), the laboratory-scale system requires extremely high spatial resolution. Range resolution is inversely proportional to the transmission bandwidth, which means that very large optical bandwidths are required. In this sense, the lab-scale work is actually more challenging than longer-range applications. The transmitter for the lab-scale system is a commercial tunable external-cavity semiconductor laser capable of producing a nominally linear frequency-swept waveform with a bandwidth of 30 nanometers (more than 1 terahertz). Compared with the RadarSat-II waveform mentioned earlier, the system can achieve range resolution on the order of 10,000 times finer.

The linearity and stability of such broadly tunable sources is quite poor, leading to significant phase error in the linear frequency-modulated waveform as well as residual amplitude modulation. This is a common dilemma: tunable sources are not highly stable, and stable sources are not generally tunable. To overcome this problem, the Aerospace team used a reference channel to directly monitor optical phase errors induced during waveform generation. These measured errors were corrected using a phase-error compensation algorithm. In the first implementations, the optical length of the reference arm was constrained to precisely match the target range. Such a constraint would seriously limit the operational utility of a SAIL system, because the reference channel would have to be retuned every time the system pointed to a new target. To remove this constraint, Aerospace developed new algorithms for intrapulse phase-error correction to handle arbitrary mismatch between the reference arm and the target path length.

Observations

The first experiment generated an image of a triangle cut from a piece of retroreflective material. The target was tilted away from the observing platform at 45 degrees to create the range depth. The resulting SAIL image displays range in the vertical direction and cross-range or azimuth in the horizontal dimension. The Aerospace reference-channel design, coupled with phase-gradient autofocus techniques, helped estimate and remove the intrapulse and pulse-to-pulse phase errors, which would render the image unrecognizable. Despite the incidence of "laser speckle" (inherent in any coherent imaging technique), the image was still highly interpretable to the human eye, and the focus was reasonably good. This initial result indicated that synthetic-aperture ladar imaging is possible despite the instability of the laser waveform (see panel 1).

The next experiment sought to demonstrate the possibility of achieving focused imagery in the presence of large waveform errors regardless of range mismatch in the reference channel. (Previous work at the Naval Research Laboratory had demonstrated a SAIL image, but the implementation required exact matching of the reference and target arms.) A discrepancy of approximately 1 meter between the length of the reference channel path and the length of the target channel path was introduced. A specialized digital compensation technique resulted in a well-focused image of the triangle target. This phase of the experiment demonstrated that SAIL can employ a fiber-optic reference channel to produce focused 2-D images even in cases where the range to target is either uncertain or variable (as in a system that points to various targets at differing ranges) (see panel 2).

In the course of their experiments, the researchers noticed very faint artifacts in the imagery. Logarithmic representations of image intensity revealed "ghost" images above and below the main image. These ghosts were traced to residual amplitude-modulation ripple in laser intensity. (With the source of the artifacts identified, researchers can now develop an algorithm to correct for them.) These results demonstrated a move beyond simply "making pictures" to doing detailed image-quality analysis (see panel 3).

Researchers then experimented with a larger, more complex target made of the same patterned retroreflective material as the triangle target. A transparency was placed in front of the target to serve as a crude "phase-screen" between the target and the transceiver. The new target was also larger than the diffraction-limited illuminating spot size, so the image had to be formed by scanning the laser spot in five strips across the target and tiling the results. The image quality was somewhat degraded because of the transparency, but the pattern of the retroreflective material was clearly visible (see panel 4).

The final SAIL experiment used a target with both a diffuse (non-retroreflecting) surface and a specular surface. With the target leaning away at 45 degrees, the diffuse-scattering surface appears bright, but the specular surface returns very little light to the receiver because it is reflected away. The image was reasonably well focused, and smooth edges in the target were just visible. This image represented the first optical synthetic-aperture image of a diffuse-scattering object (see panel 5).

Conclusion

These first laboratory steps demonstrated the proof of concept for SAIL imagery and will allow Aerospace to develop refinements to the signal processing algorithms. Of course, many real-world complications will arise in transferring these techniques to airborne or spaceborne platforms. For example, because SAIL is inherently a narrow-field-of-view technique (like looking down a soda straw), real-world implementations will require robust methods for tiling many small patches to form large composite images. Other concerns include atmospheric turbulence, unmodeled platform motion, target motion, and pointing control. The next step, currently under way, is the development of a rooftop test bed to explore some of these issues. In conjunction with the SAIL project, Aerospace has developed a balanced, phase-quadrature laser vibrometer to monitor line-of-sight optical phase errors during the SAIL image formation process.

The Defense Advanced Research Projects Agency and the Air Force Research Laboratory have initiated a program called SALTI (synthetic-aperture ladar tactical imaging) aimed at a proof-of-concept airborne demonstration to generate high-resolution 2-D and 3-D SAIL imagery combining the interpretability of electro-optical imaging, the long-range day-or-night access of high-altitude X-band synthetic-aperture radar, and the exploitability of 3-D ladar. SAIL has also been proposed for imaging and mapping planets such as Mars.